Laser power and energy sensor utilizing anisotropic thermoelectric material

ABSTRACT

A laser-radiation sensor includes a copper substrate on which is grown an oriented polycrystalline buffer layer surmounted by an oriented polycrystalline sensor-element of an anisotropic transverse thermoelectric material. An absorber layer, thermally connected to the sensor-element, is heated by laser-radiation to be measured and communicates the heat to the sensor-element, causing a thermal gradient across the sensor-element. Spaced-apart electrodes in electrical contact with the sensor-element sense a voltage corresponding to the thermal gradient as a measure of the incident laser-radiation power.

PRIORITY CLAIM

This application claims priority of U.S. Provisional Application No.61/709,060, filed Oct. 2, 2012, the complete disclosure of which ishereby incorporated herein by reference.

TECHNICAL FIELD OF THE INVENTION

The present invention relates in general to laser-radiation detectors.The invention relates in particular to a laser-radiation detector thatutilizes a transverse thermoelectric effect.

DISCUSSION OF BACKGROUND ART

Laser-radiation detectors (sensors) are used in laser applicationswherein laser-radiation power needs to be measured or monitored. Thepower measurement may be required from simple record-keeping or as partof some closed loop control arrangement. Commonly used radiationdetectors are based on either photodiodes or thermopiles.

The photodiode-based sensors detect laser-radiation by converting photonenergy of radiation to be measured into an electron-hole pairs in thephoto-diode thereby generating a corresponding current, which is used ameasure of laser-radiation power. Photodiodes sensors have a relativelyfast temporal response, with rise times typically less than 1microsecond (μs). A disadvantage of photodiode detectors is a limitedspectral response. This spectral response is determined by theparticular semiconductor materials used for forming the photodiode. Byway of example, photodiode sensors based on silicon have a spectralacceptance bandwidth between about 0.2 micrometers (μm) and about 2.0μm. A second limitation of a photodiode is relatively low optical powersaturation. Photodiodes are typically limited to direct measurement oflaser powers of less than 100 milliwatts (mW).

Thermopile sensors include a solid element which absorbs the radiation,thereby heating the element. One or more thermocouples in contact withthe element create a current or voltage representative of thelaser-radiation power incident on the element. Thermopile sensors have aslow response time relative to photodiode detectors. The response timeis dependent on the size of the sensor-element. By way of example radialthermopiles with apertures of 19 millimeters (mm) and 200 mm haveresponse times of approximately 1 second and 30 seconds respectively.Spectral response of the thermopile sensors depends on the absorptionspectrum of the sensor. With a suitable choice and configuration of thesensor, the spectral response can extend from ultraviolet (UV)wavelengths to far infrared wavelengths. With a sufficient heat sink,thermopile sensors can measure lasers power up to about 10 kilowatts(kW).

One relatively new detector type which has been proposed to offer atemporal response comparable to a photodiode detector and a spectralresponse comparable with a thermopile detector is based on using a layerof an anisotropic transverse thermoelectric material as a detectorelement. Such an anisotropic layer is formed by growing the material inan oriented polycrystalline crystalline form, with crystals inclinednon-orthogonally to the plane of the layer.

The anisotropic layer absorbs radiation to be measured thereby heatingthe layer. This creates a thermal gradient through the anisotropicmaterial in a direction perpendicular to the layer. This thermalgradient, in turn, creates an electric field orthogonal to the thermalgradient. The electric field is proportional to the intensity ofincident radiation absorbed. Such a detector may be referred to as atransverse thermoelectric effect detector. If the anisotropic layer ismade sufficiently thin, for example only a few micrometers thick, theresponse time of the detector will be comparable with that of aphotodiode detector. Spectral response is limited only by the absorbanceof the anisotropic material. A disadvantage is that the transversethermoelectric effect is relatively weak compared to the response of aphotodiode.

One transverse-thermoelectric-effect detector is described in U.S. Pat.No. 8,129,689, granted to Takahashi et al. (hereinafter Takahashi).Takahashi attempts to offset the weakness of the transversethermoelectric effect by providing first and second anisotropic materiallayers which are grown on opposite sides of a transparent crystallinesubstrate. In the Takahashi detector, radiation not absorbed by thefirst layer of anisotropic material is potentially absorbed by thesecond layer. It is proposed that a reflective coating can be added tothe second layer to reflect any radiation not absorbed by the secondlayer to make a second pass through both layers.

Oriented polycrystalline layers can be deposited by a well-knowninclined substrate deposition (ISD) process. This process is describedin detail in U.S. Pat. No. 6,265,353 and in U.S. Pat. No. 6,638,598.Oriented polycrystalline layers have also been grown by a (somewhat lessversatile) ion-beam assisted deposition (IBAD) process. One descriptionof this process is provided in a paper “Deposition of in-plane texturedMgO on amorphous Si ₃ N ₄ substrates by ion-beam-assisted deposition andcomparisons with ion-beam assisted deposited yttria-stabilized-zirconia”by C. P. Wang et.al, Applied Physics Letters, Vol 71, 20, pp 2955, 1997.

The above described Takahashi detector allows the anisotropic materiallayers to remain thin, while increasing the amount of light absorbed,but requires a transparent crystalline substrate polished on both sides,at costs potentially prohibitive for most commercial applications.Further, the Takashi detector arrangement isolates the crystallinesubstrate limiting the ability to heat-sink the substrate. This limitsthe power-handling capability of the detector to a maximum power of lessthan about 10 Watts (W), and may lead to a non-linear response.

SUMMARY OF THE INVENTION

In one aspect, a radiation detector sensor in accordance with thepresent invention comprises a substrate of a highly thermally conductivematerial. An oriented polycrystalline buffer-layer is deposited on asurface of the substrate. The buffer-layer has a crystal-orientation ata first angle between about 10 degrees and about 45 degrees. Formed ontop of the buffer is an oriented polycrystalline sensor element of athermoelectric material selected from the group of thermoelectricmaterials consisting of dysprosium barium cuprate, strontium sodiumcobaltate, and strontium cobaltate is deposited on the buffer layer. Thesensor-element has a crystalline c-axis orientation at a second anglebetween about 10-degrees and about 45-degrees relative to the normal tothe surface of the substrate. A radiation-absorber layer is provided,the radiation-absorber absorber layer being in thermal communicationwith the sensor layer. First and second electrodes are spaced apart inelectrical contact with the sensor-layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, schematically illustrate a preferredembodiment of the present invention, and together with the generaldescription given above and the detailed description of the preferredembodiment given below, serve to explain principles of the presentinvention.

FIG. 1 is a cross-section view schematically illustrating a preferredembodiment of a transverse thermoelectric detector in accordance withthe present invention, including a copper substrate, a buffer layer onthe substrate a sensor layer on the buffer layer, a protective layer onthe sensor layer, and absorber layer on the protective layer, withspaced apart electrodes in electrical contact with the sensor layer.

FIG. 2 is a plan-view from above schematically illustrating a preferredarrangement of electrodes and patterned sensor layer material for thedetector of FIG. 1.

FIG. 3 is a graph schematically illustrating measured transversethermoelectric signal as a function of incident CW laser-radiation powerfor an example of the detector of FIG. 2 wherein the sensor layer is alayer of dysprosium barium cuprate.

FIG. 4 is a graph schematically illustrating measured peakthermoelectric voltage and reflected energy as a function of incident10-nanosecond pulsed-energy for the detector example of FIG. 3.

FIG. 5 is a graph schematically illustrating a transverse thermoelectricvoltage signal as a function of time for the detector example of FIG. 4in response to irradiation by single 10-nanosecond pulse.

FIG. 6 is a contour plot schematically illustrating normalized spatialuniformity of efficiency in the detector example of FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, wherein like components are designated bylike reference numerals, FIG. 1 schematically illustrates a preferredembodiment 30 of a transverse thermoelectric sensor in accordance withthe present invention. Sensor 30 includes a substrate 32 of a highlythermally conductive material. A preferred material for substrate 32 iscopper (Cu). Copper is a preferred material due to its high thermalconductivity and relatively low cost. Substrate 32 has a polishedsurface 32A, preferably having a RMS roughness less than about 0.5 μm.The substrate is optionally in contact with a heat-sink 48, which can bepassively or actively cooled.

An oriented polycrystalline buffer-layer 34 is deposited on a surface32A of the substrate. A preferred material for buffer layer 34 ismagnesium oxide (MgO). Other suitable buffer layer materials includeyttrium stabilized zirconia (YSZ), cerium oxide (CeO₂). Buffer layer 34has a columnar grain structure with crystal-axis (the c-axis) 46 thereoftilted at an angle α in the direction by between about 10-degrees andabout 45-degrees relative to a normal 47 to substrate surface 32A. Inthe drawing, the a-c plane of the crystal axes is in the plane of thedrawing with the crystalline b-axis perpendicular to the plane of thedrawing. A preferred thickness for the buffer layer is between about 0.5μm and about 3.0 μm.

A layer 36 of sensor-material 36 is deposited on buffer layer 32. Theinclined oriented crystal structure of the buffer layer causes the layerof sensor-material to grow in the inclined polycrystalline formnecessary for providing the desired transient thermoelectric effect. Thetilted crystalline structure is indicated I the drawing by long-dashedlines.

The use of the buffer eliminates a need for the substrate to becrystalline, allowing the use of the preferred copper substrate. Thecrystalline orientation of the sensor layer (c-axis orientation) iscomparable to that of the buffer layer, i.e., between about 10 degreesand about 45 degrees but more probably between about 15-degrees andabout 40-degrees. The inclination angles for the buffer and sensorlayers can be about the same or somewhat different angles within thestated ranges.

The material of the sensor-layer is a material selected from the groupof thermoelectric materials consisting of dysprosium barium cuprate(DyBa₂Cu₃O₇-d, often abbreviated to DyBCO), strontium sodium cobaltate(Sr_(0.3)Na_(0.2)CoO₂), and strontium cobaltate (Sr₃Co₄O₉). Dysprosiumbarium cuprate is most preferred. A preferred thickness for sensor layer36 is between about 5 nanometers (nm) and about 500 nm. This thicknessis less than that of the buffer layer and is required for creating ahigh thermal gradient across the sensor layer.

Optionally, a layer 50 is deposited for protecting the sensor layer fromenvironmental degradation. Such a protection layer is critical whenDyBCO is used for sensor layer 36. Preferred materials for theprotection layer include MgO, and silicon dioxide (SiO₂). In the absenceof a protective layer, the thermoelectric properties of DyBCO willdegrade over a relatively quick time with exposure to ambient oxygen andelevated temperatures. Similarly, strontium cobaltate and strontiumsodium cobaltate are degraded by exposure to atmospheric humidity. Apreferred thickness for protective layer 50 is between about 0.2 μm andabout 2.0 μm.

An optically black radiation-absorbing layer 42 is grown on protectivelayer 50. The absorption spectrum of this layer essentially determinesthe spectral response of the inventive transverse thermoelectricradiation sensor. Suitable materials for layer 42 include boron carbide,titanium nitride, chromium oxide, gold black, or carbon. The absorptionlayer preferably has a thickness between about 0.5 μm and about 5.0 μm.Whatever the selected material, layer 42 is preferably made sufficientlythick such that about 95% or greater of radiation is absorbed andconverted to heat within the absorption layer. Incomplete absorption inlayer 42 results in less than optimum thermoelectric response signal,and can result in a non-linear response.

When the radiation-absorber layer is heated by incident radiation athermal gradient is formed across sensor layer 36 between theradiation-absorber layer and copper substrate 32. Because of a highanisotropy of the thermoelectric properties of sensor layer 36 resultingfrom the tilted crystal-axis, heat flow across the thickness of thesensor layer, generates an electric field in the sensor layerperpendicular (transverse to) to the heat-flow (thermal-gradient)direction. This transverse electric field results from significantlydifferent values of Seebeck coefficients in the crystalline a-b and cdirections for the sensor-layer material.

Elongated electrodes 38 and 40, parallel to each other and spaced apart,are deposited on sensor layer 36 in electrical contact therewith.Suitable materials for the electrodes include gold (Au), platinum (Pt),silver (Ag), and palladium (Pd). The transverse electric field betweenthe electrodes results in a voltage between, the electrodes, linearlyproportional to the incident radiation power on the absorbing layer.This voltage can be approximated by an equation:

$\begin{matrix}{V_{x} = {\frac{L}{2t}\Delta \; {T_{z}\left( {S_{ab} - S_{c}} \right)}{\sin \left( {2\alpha} \right)}}} & (1)\end{matrix}$

where V_(x) is the voltage produced between the first electrode 38 andthe second electrode 40; t is the thickness of sensor-layer 36, ΔT_(z)is the temperature differential across sensor layer 36; α is the tiltangle of the crystalline c-axis of layer 36; S_(ab) and S_(c) are theSeebeck coefficients in respectively the a-b and c crystal directions ofthe sensor layer; and L is the diameter of the incident beam of laserradiation.

FIG. 2 is plan-view from above schematically a preferred arrangement ofsensor layer 36 in which the sensor layer is patterned into a pluralityof strips 36A, each thereof extending between electrodes 38 and 40. Thewidth of the strips is designated as W₁ and the width of the gapsbetween the strips is designated W₂. Here, the strips are alignedparallel to the c-axis direction of the sensor layer. The strips can beformed by photolithography and wet-etching of a continuous layer ofthermoelectric material. Layer 36 can be defined for purposes of thisdescription and the appended claims as a sensor-element, which termapplies to continuous sensor-layer and or a layer patterned into theparallel strips of FIG. 2 or some other pattern.

In one example of the inventive detector, strips (c-axis aligned) ofDyBCO having a width W₁ of about 300 μm, with gaps W₂ of about 50 μmtherebetween, with a length between electrodes of about 33 mm and awidth of about 32 mm across the pattern of strips, provided athermoelectric signal of about 100 microvolts (μV) when the detector wasirradiated by carbon dioxide (CO₂) laser-radiation having a power ofabout 100 Watts (W). Without patterning, i.e., with sensor-element 36 asa continuous sheet between the electrodes, the thermoelectric signalvoltage was about 35 μV.

In another example of the inventive detector, with dimensions as in theabove example, but with strips 36A aligned at 45-degrees to the c-axisdirection, the thermoelectric signal was about 60 μV. In yet anotherexample, with 45-degree aligned strips, but with W₁ and W₂ each about100 μm, the thermoelectric signal was about 61 μV. These exemplaryresults indicate that, for a given active area of the detector, thethermoelectric signal is dependent on the alignment of sensor-materialstrips with the crystalline c-axis of the thermoelectric material, butmay not be sensitive to the width of the strips and gaps therebetween.Indeed, strip-width to gap ratios from 1 to 6 were tested with nosignificant change observed in thermoelectric response.

FIG. 3 is a graph schematically illustrating thermoelectric signalvoltage as a function of incident CW CO₂ laser power for an example ofthe inventive detector having a DyBCO sensor-element patterned asdepicted in FIG. 2. Again, the active area is 33 mm×32 mm. It can beseen by comparing individual data points (circles) with the best-fitstraight line that the sensor response is very linear.

FIG. 4 is a graph schematically illustrating peak thermoelectric voltage(circles) and reflected energy (diamond) as a function of incidentpulse-energy for the detector example of FIG. 3 responsive to incident10 nanosecond (ns) pulses from a 1064-nm solid state laser. The solidstraight line in the graph of FIG. 4 is a best-fit to the circle(peak-voltage) data-points, indicating the same high degree of linearityof response experienced with CW radiation as in the graph of FIG. 3.

FIG. 5 is a graph schematically illustrating thermoelectric signal as afunction of time for one of the pulses of the graph of FIG. 4. Theresponse-time (rise-time) of the signal is about 640 nanoseconds, whichis comparable to the response of a photodiode detector.

The above-described patterning of sensor layer 36 not only improvessensitivity of the inventive detector but also the spatial uniformity ofthe sensitivity. Normalized spatial distribution of sensitivity of thedetector of FIGS. 3 and 4 is schematically depicted in FIG. 6. It can beseen that the spatial uniformity over most of the useful area of thedetector is about ±5%. The spatial uniformity for the same detectorwithout a patterned sensor layer was about ±20% over the same region.

Regarding power-handling capability of the inventive detector, for anyparticular substrate and buffer layer, this will be determined by theselection of the sensor-layer material. By way of example, cuprates,such as dysprosium barium cuprate, have a maximum service temperature of≦350° C. Based on heat Transfer calculations it is estimated that adetector using dysprosium barium cuprate as a sensor-material will belimited to measuring radiation power up to about 2 kilowatts (kW).Cobaltate transverse thermoelectric materials, such as strontiumcobaltate, in principle have service temperatures ≧350° C. and shouldallow measurement of laser power greater than 2 kW.

In summary, the present invention is described in terms of a preferredand other embodiments. The invention is not limited, however, to theembodiments described and depicted herein. Rather the invention islimited only by the claims appended hereto.

What is claimed is:
 1. A laser-radiation sensor, comprising: a copper substrate; an oriented polycrystalline buffer-layer deposited on a surface of the substrate, the buffer-layer having a crystal-orientation at a first angle between about 10 degrees and about 45 degrees relative to a normal to the surface of the substrate; an oriented polycrystalline sensor-element of a thermoelectric material selected from the group of thermoelectric materials consisting of dysprosium barium cuprate, strontium sodium cobaltate, and strontium cobaltate deposited on the buffer layer, the sensor-element having a crystalline c-axis orientation at a second angle between about 10 degrees and about 45 degrees relative to the normal to the surface of the substrate; a radiation-absorber layer in thermal communication with the sensor-element; and first and second elongated electrodes spaced apart in electrical contact with the sensor-element.
 2. The laser-radiation sensor of claim 1, wherein the sensor-element is a continuous layer of the oriented polycrystalline sensor-material extending between the first and second electrodes.
 3. The laser-radiation sensor of claim 1, wherein the sensor-element includes a plurality of strips of the oriented polycrystalline sensor-material spaced apart, parallel to each other, and extending between the first and second electrodes.
 4. The laser-radiation sensor of claim 3, wherein the strips of the sensor-element are aligned parallel to the crystalline c-axis of the oriented polycrystalline sensor-material.
 5. The laser-radiation sensor of claim 1, further including a protection layer between the sensor-element and the radiation-absorber layer.
 6. The laser-radiation sensor of claim 5, wherein the protection layer is a layer of one of magnesium oxide, and silicon dioxide.
 7. The laser-radiation sensor of claim 6, wherein the radiation-absorber layer is a layer of a radiation-absorbing material selected from a group of radiation-absorbing materials consisting of boron carbide, titanium nitride, chromium oxide, gold black, and carbon.
 8. The laser-radiation sensor of claim 1, wherein the electrodes include a metal selected from a group of metals consisting of gold, platinum, silver, and palladium.
 9. The laser- radiation sensor of claim 1, wherein the buffer layer is a layer of material selected from a group of materials consisting of magnesium oxide, yttrium stabilized zirconia, and cerium oxide.
 10. The laser-radiation sensor of claim 1, wherein the first and second angles are about the same.
 11. A laser-radiation sensor, comprising: a substrate of a highly thermally conductive material; an oriented polycrystalline buffer-layer deposited on a surface of the substrate, the buffer-layer having a crystal-orientation at a first angle between about 10 degrees and about 45 degrees relative to a normal to the surface of the substrate; an oriented polycrystalline sensor-element of a thermoelectric material selected from the group of thermoelectric materials consisting of dysprosium barium cuprate, strontium sodium cobaltate, and strontium cobaltate deposited on the buffer layer, the sensor-element having a crystalline c-axis orientation at a second angle between about 10 degrees and about 45 degrees relative to the normal to the surface of the substrate; a protection layer deposited on the sensor-element; a radiation-absorber layer deposited on the protection layer; first and second elongated electrodes spaced apart in electrical contact with the sensor-element; and wherein the sensor-element includes a plurality of strips of the oriented polycrystalline sensor-material spaced apart, parallel to each other, and extending between the first and second electrodes, with each of the strips in electrical contact with the first and second electrodes.
 12. The laser-radiation sensor of claim 11, wherein the substrate is a copper substrate.
 13. The laser-radiation sensor of claim 11, wherein the buffer layer has a thickness between about 0.5 micrometers and about 3.0 micrometers, and is a layer of material selected from a group of materials consisting of magnesium oxide, yttrium stabilized zirconia, and cerium oxide.
 14. The laser-radiation sensor of claim 11, wherein the strips of sensor material have a thickness between about 5 nanometers and about 500 nanometers.
 15. The laser-radiation sensor of claim 11, wherein the protection layer has a thickness of between about 0.2 micrometers and about 2.0 micrometers, and is a layer of one of magnesium oxide, and silicon dioxide.
 16. The laser-radiation sensor of claim 12, wherein the absorber layer has a thickness of between about 0.5 micrometers and about 5.0 micrometers, and is a layer of a radiation-absorbing material selected from a group of radiation-absorbing materials consisting of boron carbide, titanium nitride, chromium oxide, gold black, and carbon. 